Investigations of the catalytic mechanisms of the following enzymes: NADPH-sulfite reductase of Escherichia coli, ferredoxin-sulfite reductase of spinach, ferredoxin-nitrite reductase of spinach, and sulfite oxidase of bovine liver, will be investigated utilizing both static and rapid kinetic approaches with analysis of samples by EPR and optical spectroscopy. A possible role for siroheme, the prosthetic group of the first three enzyme named, in animal systems will be explored.